“At the time we were working on our study there were one or two reports out there describing people who had vaped and had sterile pneumonias,” said senior study senior author Dr. Farrah Kheradmand, a professor of medicine, pathology and immunology at the Baylor College of Medicine. “The reports showed staining of cells within the lung that looked identical to what our mice had.”

What Kheradmand’s mice had was immune cells in the lungs that had become clogged up with fat.

To get a better understanding of how e-cigarettes might affect lung health, Kheradmand and her colleagues studied the impact of both traditional cigarette smoke and e-cigarette vapor on three groups of mice. A fourth group that was exposed only to air was used as a control.

The mice in the e-cigarette group were exposed to either vapor containing nicotine dissolved in the common vaping solvents of propylene glycol and vegetable glycerin, or vapor that came just from the solvents, without the nicotine.

The exposure to smoke, vapor or air lasted four months, which would be comparable to years of smoking in humans, Kheradmand said.

As expected, mice exposed to cigarette smoke developed emphysema, whereas the mice exposed to vapor did not.

But that didn’t mean the vapor-exposed mice were in the clear. When Kheradmand and her colleagues examined immune cells from the mouse lungs, they found a striking abnormality: The cells were clogged with fat. There were no such changes in the other mice.

Immune cells isolated from the lungs of mice exposed to chronic vape (ENDS), but not air or smoke, show large abnormal translucent bodies inside the cells (arrows). Lower images show that the abnormal translucent bodies are full of fat. The study was funded by the NIEHS.ASCI, reproduced with permission from the Journal of Clinical Investigation

The researchers initially thought the fat particles, or lipids, filling up the immune cells had come directly from the vegetable glycerin, which is a type of fat. But as it turned out, the fat had come from the animals’ own lungs.

The fluid lining the lungs contains proteins and lipids, Kheradmand said. While it’s normal for the immune cells to take up some this fat, when the mice were exposed to e-cigarette vapor, the cells became overwhelmed, and the accumulation went into overdrive.

This affected the cells’ ability to defend against pathogens, Kheradmand said. In the second part of the study, the vapor-exposed mice were also exposed to influenza and developed more severe infections.

“When those mice were exposed to a virus that normally doesn’t kill mice, they were not capable of handling the virus,” she added.

While the study is only in mice, which means the findings can’t be directly applied to people, Kheradmand said. “I cannot imagine the process in humans would be very different.”

Other experts agreed.

“With an animal study there is an uncertainty of how it applies to humans,” said Dr. Ravi Kalhan, director of the Asthma and COPD Program at the Northwestern University Feinberg School of Medicine. “That said, it’s really intriguing that e-cigarette vapor could cause a change in the fundamental ability of the lungs to respond to pathogens.”

The study underscores how little is known about e-cigarettes, Kalhan added.

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“This speaks to how we don’t really know what the effects from these delivery systems are, beyond nicotine,” he said. A small study, published in July, found that a single session of inhaling nicotine-free vapor caused changes in blood flow in humans.

“The chemicals used in e-cigarettes have never been studied as inhalants,” said Dr. Panagis Galiatsatos, an assistant professor of medicine and director of the Tobacco Treatment Clinic at the Johns Hopkins School of Medicine. “They are approved only to be ingested or given through an IV.”

The new research suggests that “we really need to take a big pause" on e-cigarettes, Galiatsatos said.

The study’s findings are important especially in light of the fact that e-cigarettes have been marketed as safer than traditional cigarettes, said Dr. Janet Lee, a professor of medicine and director of the Acute Lung Injury Center of Excellence at the University of Pittsburgh. While the findings are in mice rather than humans, “this may be the first clue that there is something going on with lipid homeostasis,” she said. “And it shows that [vaping] may impair the function of immune cells.”

Your lungs were designed to inhale air, Kalhan said, not vapor. The idea that vaping could “enhance our susceptibility to viruses is really scary,” he added.

Linda Carroll is a regular health contributor to NBC News and Reuters Health. She is coauthor of "The Concussion Crisis: Anatomy of a Silent Epidemic" and "Out of the Clouds: The Unlikely Horseman and the Unwanted Colt Who Conquered the Sport of Kings."